Wavelength conversion system, optical integrated device and wavelength conversion method

ABSTRACT

A wavelength conversion system includes a Mach-Zehnder interferometer including two optical waveguides, a non-linear medium provided on one of the two optical waveguides, and a branching ratio adjuster for adjusting the branching ratio of multiplexed light produced by multiplexing signal light and pumping light so that the powers of the signal light and the pumping light which are to be emitted from the two optical waveguides are equal to each other. The multiplexed light whose branching ratio is adjusted by the branching ratio adjuster is introduced into the two optical waveguides such that the non-linear medium generates phase conjugation light of the signal light and the light guided through the one optical waveguide and the light guided through the other one of the two optical waveguides interfere with each other so that the phase conjugation light is extracted as wavelength conversion light.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to JapaneseApplication No. 2005-258318 filed on Sep. 6, 2005 in Japan, the contentsof which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a wavelength conversion system, an opticalintegrated device and a wavelength conversion method.

(2) Description of the Related Art

As increase in speed and capacity of optical communication proceeds, anall-optical signal processing technique which involves no conversion ofan optical signal into an electric signal to perform processing isdemanded.

Thanks to the progress of wavelength division multiplexing (WDM)techniques in recent years, it is possible to fully use an entirewavelength bandwidth over several THz of silica glass optical fibers. Inthe WDM wherein individual ones of wavelengths of light transmitdifferent information applied thereto, wavelength conversion forimplementing prevention of wavelength conflict and exchange bywavelength routing between sub networks is an essentially requiredtechnique.

When it is tried to perform all-optical wavelength conversion, methodswhich utilize a non-linear medium (NLM) such as an optical fiber or asemiconductor are used frequently. Among the methods, a wavelengthconversion method which uses a non-linear effect of a semiconductoroptical amplifier (SOA) is studied actively because it allowsminiaturization of a system and can provide a high non-linear effectwith low power consumption.

All-optical wavelength conversion methods which use the SOA can beclassified into those of the optical switch type which make use of crossgain modulation (XGM) or cross phase modulation (XPM) and those of thecoherent type which use difference frequency generation (DFG) or fourwave mixing (FWM).

Of the two methods, the wavelength conversion method of the coherenttype is ready also for a modulation format such as, for example,differential phase shift keying (DPSK) because it can perform very highspeed wavelength conversion due to its high non-linear responsibilityand besides maintains phase information also after the wavelengthconversion.

Where the wavelength conversion method which uses the DFG and thewavelength conversion method which uses the FWM are compared with eachother, in the wavelength conversion method which uses the DFG which is asecond-order non-linear effect, the distance between wavelengths oflight which act with each other is greater than that in the conversionwavelength method which uses the FWM which makes use of a third-ordernon-linear effect. Therefore, the wavelength conversion method whichuses the FWM is more advantageous from the point of view of the facilityin establishment of phase matching.

It is to be noted that Japanese Patent Laid-Open No. 2000-250081 andJapanese Patent Laid-Open No. 2004-185021 were found through a prior artsearch conducted.

SUMMARY OF THE INVENTION

Incidentally, in the wavelength conversion method which uses the FWM,the phenomenon that, if signal light ω_(s) and pumping light ω_(p) areinputted to an optical waveguide made of a non-linear medium, then phaseconjugate light ω_(c) (=2ω_(p)−ω_(s)) of the signal light is produced asseen in FIG. 6 is utilized to extract the phase conjugate light ω_(c)generated by the non-linear medium as wavelength conversion light.

However, also the signal light ω_(s) and the pumping light ω_(p) areoutputted together with the phase conjugate light ω_(c).

Therefore, in order to extract only the phase conjugate light ω_(c) asthe wavelength conversion light, a filter is used (for example, avariable wavelength filter is provided externally) to cut the signallight ω_(s) and the pumping light ω_(p).

Meanwhile, in such a wavelength conversion method as described above, inorder to convert signal light (input light) of a certain wavelength intosignal light (wavelength conversion light) of an arbitrary wavelength(that is, in order to adjust the wavelength of the phase conjugate lightω_(c) of an arbitrary wavelength as wavelength conversion light), thewavelength of the pumping light ω_(p) should be changed.

However, where a filter is used, the width in wavelength conversion islimited by the band of the filter. Further, another filter of adifferent band is sometimes required in response to the modulation speedof a signal. On the other hand, if a large number of filters areprovided in order to extract phase conjugate light ω_(c) of an arbitrarywavelength as wavelength conversion light, then the number of partsincreases as much. Further, such provision of a large number of filtersis disadvantageous in achievement of miniaturization and integration ofapparatus.

Further, where a variable wavelength filter is used, also it isnecessary to sweep the center wavelength of the filter, and the width inwavelength conversion is limited also by the wavelength sweep range.Particularly since the wavelength sweep range is determined by thecharacteristic of a filter including a sweeping mechanism, actual use ofthe filter is restricted by various factors. Further, the wavelengthconversion speed is restricted by operation of the filter.

It is an object of the present invention to provide a wavelengthconversion system, an optical integrated device and a wavelengthconversion method by which only wavelength conversion light can beextracted without using a filter.

In order to attain the object described above, according to an aspect ofthe present invention, there is provided a wavelength conversion system,comprising a Mach-Zehnder interferometer including two opticalwaveguides, a non-linear medium provided on one of the two opticalwaveguides, and a branching ratio adjustment section for adjusting thebranching ratio of multiplexed light produced by multiplexing signallight and pumping light so that the powers of the signal light and thepumping light which are to be emitted from the two optical waveguidesare equal to each other, the multiplexed light whose branching ratio isadjusted by the branching ratio adjustment section being introduced intothe two optical waveguides such that the non-linear medium generatesphase conjugation light of the signal light and the light guided throughthe one optical waveguide and the light guided through the other one ofthe two optical waveguides interfere with each other so that the phaseconjugation light is extracted as wavelength conversion light.

According to another aspect of the present invention, there is provideda wavelength conversion system, comprising a Mach-Zehnder interferometerincluding two optical waveguides, a first non-linear medium provided onone of the two optical waveguides, and a second non-linear mediumprovided on the other one of the two optical waveguides and having anon-linear susceptibility different from that of the first non-linearmedium, multiplexed light produced by multiplexing signal light andpumping light being branched and introduced into the two opticalwaveguides such that the first and second non-linear media individuallygenerate phase conjugation light of the signal light and the lightguided through the one optical waveguide and the light guided throughthe other one of the two optical waveguides interfere with each other sothat the phase conjugation light is extracted as wavelength conversionlight.

According to a further aspect of the present invention, there isprovided an optical integrated device, comprising a pre-stageMach-Zehnder interferometer including two optical waveguides forbranching and guiding multiplexed light produced by multiplexing signallight and pumping light, a phase shifter provided on at least one of thetwo optical waveguides which form the pre-stage Mach-Zehnderinterferometer, a post-stage Mach-Zehnder interferometer including aninput side coupler, an output side coupler, and two optical waveguidesfor connecting the input side coupler and the output side coupler toeach other, and a semiconductor optical amplifier provided on one of thetwo optical waveguides which form the pre-stage Mach-Zehnderinterferometer, the multiplexed light whose branching ratio is adjustedby the pre-stage Mach-Zehnder interferometer and the phase shifter sothat the powers of the signal light and the pumping light to beindividually emitted from the two optical waveguides which form thepost-stage Mach-Zehnder interferometer are equal to each other beingintroduced into the two optical waveguides which form the post-stageMach-Zehnder interferometer such that the semiconductor opticalamplifier generates phase conjugation light of the signal light and thelight guided through the one optical waveguide which forms thepost-stage Mach-Zehnder interferometer and the light guided through theother one of the two optical waveguides which form the post-stageMach-Zehnder interferometer interfere with each other so that the phaseconjugation light is extracted as wavelength conversion light.

According to a still further aspect of the present invention, there isprovided an optical integrated device, comprising a Mach-Zehnderinterferometer including an input side coupler, an output side coupler,and two optical waveguides for connecting the input side coupler and theoutput side coupler to each other, a first semiconductor opticalamplifier provided on one of the two optical waveguides, and a secondsemiconductor optical amplifier provided on the other one of the twooptical waveguides and having non-linear susceptibility different fromthat of the first semiconductor optical amplifier, multiplexed lightproduced by multiplexing signal light and pumping light being branchedand introduced into the two optical waveguides such that the first andsecond semiconductor optical amplifiers individually generate phaseconjugation light of the signal light and the light guided through theone optical waveguide and the light guided through the other one of thetwo optical waveguides interfere with each other so that the phaseconjugation light is extracted as wavelength conversion light.

According to a yet further aspect of the present invention, there isprovided a wavelength conversion method, comprising the steps ofadjusting a branching ratio of multiplexed light produced bymultiplexing signal light and pumping light emitted from two opticalwaveguides which form a Mach-Zehnder interferometer so that the powersof the signal light and pumping light are equal to each other,introducing the multiplexed light whose branching ratio is adjusted intothe two individual optical waveguides, generating phase conjugationlight of the signal light by means of a non-linear medium provided onone of the two optical waveguides, and causing the light guided throughthe one optical waveguide with a gain generated by the non-linear mediumand the light guided through the other one of the two optical waveguidesto interfere with each other so that the phase conjugation light isextracted as wavelength conversion light.

According to a yet further aspect of the present invention, there isprovided a wavelength conversion method, comprising the steps ofintroducing multiplexed light produced by multiplexing signal light andpumping light into two optical waveguides which form a Mach-Zehnderinterferometer, generating phase conjugation light of the signal lightby means of a first non-linear medium provided on one of the two opticalwaveguides, generating phase conjugation light of the signal light bymeans of a second non-linear medium provided on the other one of the twooptical waveguides and having a non-linear susceptibility different fromthat of the first non-linear medium, and causing the light guidedthrough the one optical waveguide and the light guided through the otheroptical waveguide to interfere with each other so that the phaseconjugation light is extracted as wavelength conversion light.

With the wavelength conversion systems, optical integrated devices andwavelength conversion methods, there is an advantage that signal lightof an arbitrary wavelength, an arbitrary modulation speed and anarbitrary modulation format can be converted into wavelength conversionlight of an arbitrary wavelength, and only the wavelength conversionlight can be extracted without using a filter.

As a result, the number of parts can be reduced in comparison with analternative case wherein a filter is used. Also it is possible toachieve miniaturization and integration of apparatus. Further, such asituation that the width in wavelength conversion is restricted by thebandwidth or the wavelength sweeping width of a filter is eliminated.Furthermore, also such a situation that the wavelength conversion speedis restricted by operation of a filter is eliminated, and wavelengthconversion can be performed at a high speed.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description and theappended claims, taken in conjunction with the accompanying drawings inwhich like parts or elements denoted by like reference symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a wavelength conversion systemand a wavelength conversion method according to a first embodiment ofthe present invention;

FIGS. 2(A) to 2(E) are schematic views illustrating a configuration anda production method of an optical integrated device according to thefirst embodiment of the present invention;

FIG. 3 is a schematic view showing a wavelength conversion apparatusaccording to a modification to the first embodiment of the presentinvention;

FIG. 4 is a schematic view illustrating a wavelength conversion systemand a wavelength conversion method according to a second embodiment ofthe present invention;

FIGS. 5(A) to 5(E) are schematic views illustrating a configuration anda production method of an optical integrated device according to thesecond embodiment of the present invention; and

FIG. 6 is a schematic view illustrating a wavelength conversion systemwhich uses popular four wave mixing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, wavelength conversion systems, optical integrateddevices and wavelength conversion methods according to embodiments ofthe present invention are described.

[First Embodiment]

First, a wavelength conversion system, an optical integrated device anda wavelength conversion method according to a first embodiment of thepresent invention are described with reference to FIGS. 1 to 2(A) to2(E).

The wavelength conversion system according to the present embodimentuses four wave mixing (FWM) as a non-linear effect of a non-linearmedium (NLM). In particular, the wavelength conversion system makes useof the phenomenon that, if signal light ω_(s) and pumping light ω_(p)are inputted to a non-linear medium (here, a medium in which athird-order non-linear phenomenon occurs), then phase conjugate lightω_(c) (=2ω_(p−ω) _(s)) of the signal light is produced, to extract thephase conjugate light ω_(c) (phase conjugate light by a third-ordernon-linear phenomenon) produced by a non-linear medium to performwavelength conversion.

Here, the non-linear medium may be, for example, a semiconductor opticalamplifier (SOA) or a non-linear optical fiber (for example, a highnon-linear optical fiber) which has a semiconductor gain medium.

Particularly, in the present embodiment, as seen in FIG. 1, aMach-Zehnder interferometer 2 is used to extract the phase conjugatelight ω_(c) produced by the non-linear medium (NLM) 1 as wavelengthconversion light. In particular, in the present embodiment, thewavelength conversion system (apparatus) includes a Mach-Zehnderinterferometer 2 having two optical waveguides 2A and 2B as shown inFIG. 1. Further, the non-linear medium 1 is provided on one of theMach-Zehnder interferometer 2, that is, on the optical waveguide 2A suchthat light guided through the optical waveguide 2A of the Mach-Zehnderinterferometer 2 and light guided through the other optical waveguide 2Binterfere with each other so that phase conjugate light ω_(c) producedby the non-linear medium 1 is extracted as wavelength conversion light.

Here, in order to introduce multiplexed light produced by multiplexingthe signal light ω_(s) and the pumping light ω_(p) into the Mach-Zehnderinterferometer 2, a multiplexer 3 (here, a multiplexing coupler) isprovided as seen in FIG. 1. Further, a coupler 4 (here, a 2×2 coupler;branching ratio of 1:1) is provided in order to cause light propagatedalong the two optical waveguides 2A and 2B of the Mach-Zehnderinterferometer 2 to interfere with each other. In this instance, onlythe phase conjugate light ω_(c) is emitted as the wavelength conversionlight from one of output ports of the coupler 4 as seen in FIG. 1.

Further, if the multiplexed light produced by multiplexing the signallight ω_(s) and the pumping light ω_(p) is introduced into thenon-linear medium 1, then a gain is generated occasionally. The gainwill make the powers (light intensities) of the light guided through theoptical waveguide 2A, on which the non-linear medium 1 is provided, andthe light guided through the optical waveguide 2B, on which thenon-linear medium 1 is not provided, different from each other.

Therefore, in the present embodiment, in order that the powers (lightintensities) of the signal light ω_(s) and the pumping light ω_(p) to beemitted from the optical waveguides 2A and 2B of the Mach-Zehnderinterferometer 2 (two outputs of the Mach-Zehnder interferometer 2) maybe equal to each other, a branching ratio adjuster 5 for adjusting thebranching ratio of the multiplexed light produced by multiplexing thesignal light ω_(s) and the pumping light ω_(p) is provided at asucceeding stage to the multiplexer 3 and a preceding stage to theMach-Zehnder interferometer 2 as shown in FIG. 1. It is to be notedthat, where the non-linear medium 1 is of the type which generates nogain, the branching ratio should be adjusted to 1:1 by means of thebranching ratio adjuster 5.

Therefore, the branched lights whose branching ratio is adjusted by thebranching ratio adjuster 5 are introduced into the two opticalwaveguides 2A and 2B of the Mach-Zehnder interferometer 2 (two inputports of the Mach-Zehnder interferometer 2).

Here, the branching ratio adjuster 5 is a branching ratio adjustingcoupler (for example, a variable branching ratio coupler). It is to benoted that a Mach-Zehnder interferometer or the like may be used for thebranching ratio adjuster 5 as hereinafter described.

Further, in the present embodiment, a phase shifter 6 (phasecompensator) is provided on the other optical waveguide 2B (opticalwaveguide on which the non-linear medium 1 is not provided) of theMach-Zehnder interferometer 2 as shown in FIG. 1 to effect phaseadjustment so that the phases of the lights propagated through the twooptical waveguides 2A and 2B may coincide with each other. It is to benoted that the phase shifter 6 may be provided on at least one of thetwo optical waveguides 2A and 2B of the Mach-Zehnder interferometer 2.Further, if it is possible to adjust the phases to each other by someother measures such as adjustment of the lengths of the two opticalwaveguides 2A and 2B of the Mach-Zehnder interferometer 2, then thephase shifter 6 need not be provided.

Now, a wavelength conversion method which uses such a wavelengthconversion system as described above is described.

First, as seen in FIG. 1, multiplexed light produced by multiplexingsignal light ω_(s) and pumping light ω_(p) is branched and introducedinto the two optical waveguides 2A and 2B of the Mach-Zehnderinterferometer 2. There upon, the branching ratio of the multiplexedlight of the signal light ω_(s) and the pumping light ω_(p) is adjustedby means of the branching ratio adjuster 5 so that the powers (lightintensities) of the signal light ω_(s) and the pumping light p to beemitted from the optical waveguides 2A and 2B which form theMach-Zehnder interferometer 2 may be equal to each other. Then, themultiplexed lights whose branching ratio has been adjusted by thebranching ratio adjuster 5 are individually introduced into the twooptical waveguides 2A and 2B of the Mach-Zehnder interferometer 2.

When one of the multiplexed lights introduced into the optical waveguide2A until it passes the non-linear medium 1 (NLM), phase conjugate lightω_(c) of the signal light is generated by the non-linear medium 1.

Thereafter, the light propagated through the optical waveguide 2A andthe light propagated through the other optical waveguide 2B interferewith each other at the coupler 4 (at the branching ratio of 1:1). Inthis instance, the phase conjugate light ω_(c) is inputted only from theoptical waveguide 2A, on which the non-linear medium 1 is provided, toand branched at 1:1 by the coupler 4, and is outputted from the twoports of the coupler 4. On the other hand, the signal light ω_(s) andthe pumping light ω_(p) propagated through the optical waveguides 2A and2B cancel each other at the coupler 4 and are not outputted from one ofthe ports of the coupler 4. Therefore, only the phase conjugate lightω_(c) is outputted from the one port of the coupler 4, and this isextracted as wavelength conversion light. Therefore, it is consideredthat the signal light ω_(s) is converted into the phase conjugate lightω_(c).

In the present embodiment, phase shifting is performed by the phaseshifter 6 provided on the optical waveguide 2B on which the non-linearmedium 1 is not provided so that the phases of the lights propagatedthrough the two optical waveguides 2A and 2B may coincide with eachother.

Now, an optical integrated device which uses the wavelength conversionsystem described above is described with reference to FIGS. 2(A) to2(E).

It is to be noted that FIG. 2(B) is a schematic sectional view takenalong line B-B′ of FIG. 2(A); FIG. 2(C) is a schematic sectional viewtaken along line C-C′ of FIG. 2(A); FIG. 2(D) is a schematic sectionalview taken along line D-D′ of FIG. 2(A); and FIG. 2(E) is a sectionalview take along line E-E′ of FIG. 2(A).

Referring first to FIG. 2(A), the present optical integrated device isformed as a monolithic integrated device wherein a pre-stageMach-Zehnder interferometer 50 and phase shifters 51 and 52 which formthe branching ratio adjuster 5 of the wavelength conversion systemdescribed herein above, and a post-stage Mach-Zehnder interferometer 20and a semiconductor optical amplifier (SOA) 10, used as the non-linearmedium 1, which form a wavelength conversion section (the Mach-Zehnderinterferometer 2 and the non-linear medium 1 of the wavelengthconversion system described hereinabove) for removing signal light ω_(s)and pumping light ω_(p) to selectively extract phase conjugate lightω_(c) are integrated monolithically.

It is to be noted that the multiplexer 3 of the wavelength conversionsystem described above is not shown in FIG. 2(A). Further, in FIG. 2(A),reference numeral 41 denotes an input side optical waveguide forinputting multiplexed light therethrough, and reference numeral 42denotes an output side optical waveguide for outputting the phaseconjugate light ω_(c) therethrough.

The pre-stage Mach-Zehnder interferometer 50 is provided at thepreceding stage as viewed from the incoming side of light as seen inFIG. 2(A) and includes two optical waveguides 53 and 54 for branchingand guiding multiplexed light of the signal light ω_(s) and the pumpinglight ω_(p). In other words, the pre-stage Mach-Zehnder interferometer50 is composed of an input side coupler 55, an output side coupler 56,and two optical waveguides (slab waveguides) 53 and 54 forinterconnecting the input side coupler 55 and the output side coupler56.

The input side coupler 55 and the output side coupler 56 are multi-modeinterference (MMI) couplers. It is to be noted here that, while an MMIcoupler is used for the input side coupler 55 and the output sidecoupler 56 from the point of view of the controllability of thebranching characteristic, the coupler to be used here is not limited tothis, but, for example, a directional coupler or a Y-branching couplermay be used instead.

Each of the phase shifters 51 and 52 is configured such that it includesa phase shifting electrode 51A or 52A provided for a corresponding oneof the two optical waveguides 53 and 54 which form the pre-stageMach-Zehnder interferometer 50 so as to adjust the phase by injectingelectric current into the optical waveguide 53 or 54 through the phaseshifting electrode 51A or 52A as seen in FIG. 2(A). It is to be notedthat a phase shifter may be provided on at least one of the two opticalwaveguides 53 and 54 which form the pre-stage Mach-Zehnderinterferometer 50.

The post-stage Mach-Zehnder interferometer 20 is provided on thesucceeding side as viewed from the incoming side of light as seen inFIG. 2(A) and includes an input side coupler 21, an output side coupler22, and two optical waveguides (slab waveguides) 23 and 24 forinterconnecting the input side coupler 21 and the output side coupler22. It is to be noted here that the output side coupler 56 of thepre-stage Mach-Zehnder interferometer 50 described above is used also asthe input side coupler 21.

The output side coupler 22 is a multi-mode interference (MMI) coupler.It is to be noted here that, while an MMI coupler is used for the outputside coupler 22 from the point of view of the controllability of thebranching characteristic, the coupler to be used here is not limited tothis, but, for example, a directional coupler or a Y-branching couplermay be used instead.

The SOA 10 is provided on the optical waveguide 23 which form thepost-stage Mach-Zehnder interferometer 20 as seen in FIG. 2(A). The SOA10 includes an electrode 10× provided on an optical waveguide includingan active layer made of a gain medium which generates a third-ordernon-linear phenomenon such that electric current (control current) canbe injected into the active layer through the electrode 10×.

Further, in the present embodiment, multiplexed light whose branchingratio is adjusted by the pre-stage Mach-Zehnder interferometer 50 andthe phase shifters 51 and 52 so that the powers (light intensities) ofthe signal light ω_(s) and the pumping light ω_(p) to be emitted fromeach of the two optical waveguides 23 and 24 which form the post-stageMach-Zehnder interferometer 20 may be equal to each other is introducedinto the two optical waveguides 23 and 24 which form the post-stageMach-Zehnder interferometer 20. And then, the SOA 10 generates phaseconjugate light ω_(c) of the signal light and the light guided throughthe optical waveguide 23 which forms the post-stage Mach-Zehnderinterferometer 20 and the light guided through the other opticalwaveguide 24 of the post-stage Mach-Zehnder interferometer 20 interferewith each other so that the phase conjugate light ω_(c) is extracted aswavelength conversion light.

The branching ratio of the multiplexed light incoming to each of the twooptical waveguides 23 and 24 which form the post-stage Mach-Zehnderinterferometer 20 is adjusted in the following manner.

In particular, the multiplexed light produced by multiplexing the signallight ω_(s) and the pumping light ω_(p) is branched and introduced intothe two optical waveguides 53 and 54 of the pre-stage Mach-Zehnderinterferometer 50, and then, electric current is injected into the phaseshifting electrodes 51A and 52A (phase shifters 51 and 52) provided inthe pre-stage Mach-Zehnder interferometer 50 in response to a gaingenerated in the SOA 10 [that is, in response to the powers (lightintensities) of the signal light ω_(s) and the pumping light ω_(p)amplified by the SOA 10]. As a result, the phases of the multiplexedlight propagated through the optical waveguides 53 and 54 of thepre-stage Mach-Zehnder interferometer 50 are varied to adjust thebranching ratio of the multiplexed light to be introduced into theoptical waveguides 23 and 24 of the post-stage Mach-Zehnderinterferometer 20 through the output side coupler 56 of the pre-stageMach-Zehnder interferometer 50 (the output side coupler 56 serves alsoas the input side coupler 21 of the post-stage Mach-Zehnderinterferometer 20).

Further, in the present embodiment, a phase shifter (phase compensator)60 as the phase shifter 6 of the wavelength conversion system describedhereinabove is provided on the other optical waveguide 24 of thepost-stage Mach-Zehnder interferometer 20 (optical waveguide on whichthe non-linear is not provided) such that the phases of the lightspropagated through the two optical waveguides 23 and 24 can be adjustedso as to coincide with each other as seen in FIG. 2(A).

Here, the phase adjust 60 includes a phase shifting electrode 60Aprovided on the optical waveguide 24 of the post-stage Mach-Zehnderinterferometer 20 such that the phase is adjusted by injecting electriccurrent to the optical waveguide 24 through the phase shifting electrode60A. It is to be noted that the phase shifter 60 may be provided on atleast one of the two optical waveguides 23 and 24 of the post-stageMach-Zehnder interferometer 20. Further, if it is possible to adjust thephases to each other by some other measures such as adjustment of thelengths of the two optical waveguides 23 and 24 of the Mach-Zehnderinterferometer 20, then the phase shifter need not be provided.

Now, a method of producing the optical integrated device according thepresent embodiment is described suitably with reference to FIGS. 2(A) to2(E).

First, an n-InP cladding layer 61 (for example, of a thickness of 1 μmor less), a lower side SCH layer 62 (InGaAsP layer; optical guide layer;for example, of a light emission wavelength of 1.15 μm; for example, ofa thickness of 50 nm), a strained quantum well active layer 63 (having,for example, six InGaAs well layers; for example, of a strain amount of+0.8%; for example, of a thickness of 100 nm), an upper side SCH layer64 (InGaAsP layer; optical guide layer; for example, of a light emissionwavelength of 1.15 μm; for example, of a thickness of 50 nm), and ap-InP cladding layer 65 (for example, of a thickness of 300 nm) aregrown on a n-type InP substrate (n-InP substrate) 100, for example, bymetal organic chemical vapor phase epitaxy (MOVPE) to form a stackedstructure including the strained quantum well active layer 63 (SOAactive layer) which forms an SOA as seen in FIG. 2(E).

Then, an SiO₂ mask (dielectric mask) is formed only over an area overwhich the SOA 10 is to be formed (refer to FIG. 2(A), and leaving thisportion, the layers are removed, for example, by wet etching until then-InP cladding layer 61 is exposed (that is, the layers from the p-InPcladding layer 65 to the lower side SCH layer 62 are removed).

Then, as seen in FIGS. 2(B) to 2(E), at the portion from which thelayers are removed, a lower side SCH layer 62 (InGaAsP layer; opticalguide layer; for example, of a light emission wavelength of 1.15 μm; forexample, of a thickness of 50 nm), a waveguide core layer 66 (InGaAsPlayer; for example, of a light emission wavelength of 1.3 μm; forexample, of a thickness of 200 nm), an upper side SCH layer 64 (InGaAsPlayer; optical guide layer; for example, of a light emission wavelengthof 1.15 μm; for example, of a thickness of 50 nm), and a p-InP claddinglayer 65 (for example, of a thickness of 300 nm) are butt joint grown,for example, by MOVPE or the like to form a stacked structure includingthe waveguide core layer 66.

Then, an SiO₂ mask is formed at portions at which the optical waveguides23 and 24, 41, 42, 51 and 52 are to be formed, at portions at which theMMI couplers 55, 56 (21) and 22 are to be formed and at a portion atwhich the SOA 10 is to be formed [refer to FIG. 2(A) ], and then dryetching such as, for example, ICP-RIE (Inductively Coupled PlasmaReactive Ion Etching) is performed to form a waveguide mesa structure,for example, of a height of 1.5 μm and a width of 1.5 μm as seen inFIGS. 2(B) to 2(E).

Then, a p-InP current block layer 67 (first current block layer) and ann-InP current block layer 68 (second current block layer) are grown onthe opposite sides of the mesa structure, for example, by MOVPE to forma current constriction structure as seen in FIGS. 2(B) to 2(E).

After the current constriction structure is formed in this manner, theSiO₂mask is removed, and a p-InP cladding layer 69 (for example, of athickness of 3 μm) and an InGaAsP contact layer 70 (for example, of alight emission wavelength of 1.3 μm; for example, of a thickness of 100nm) are grown at an upper portion thereby to complete the epitaxialgrowth as seen in FIGS. 2(B) to 2(E).

From the wafer on which the epitaxial growth is completed in thismanner, the InGaAsP contact layer 70 is removed except at the portion atwhich the SOA 10 is to be formed and at the portions at which the phaseshifters 60, 51 and 52 are to be formed, and a SiO₂ film 71 is formed.Thereafter, the SiO₂ film 71 at the portion at which the SOA 10 is to beformed and at the portions at which the phase shifters 60, 51 and 52 areto be formed (at the portions at which the InGaAsP contact layer 70remains) is removed, and p side electrodes 51A, 52A and 60A are formedon the InGaAsP contact layer 70. Meanwhile, an n side electrode 73 isformed on the rear face of the substrate.

Finally, anti-reflection coating is applied to the opposite side facesof the device after cleavage thereby to complete the optical integrateddevice.

Accordingly, with the wavelength conversion system, optical integrateddevice and wavelength conversion method of the present embodiment, sincean interference effect is used to remove signal light ω_(s) and pumpinglight ω_(p), there is an advantage that input signal light of anywavelength, any modulation speed and any modulation format can beconverted into signal light of an arbitrary wavelength and only thewavelength conversion light can be extracted without using a filter (forexample, without external provision of a variable wavelength filter). Asa result, the number of parts can be reduced when compared with analternative arrangement wherein a filter is used.

Also it becomes possible to achieve miniaturization and integration ofapparatus. Particularly it is possible to integrate apparatusmonolithically like the optical integrated device described hereinabove, and the functions of a conventional non-linear medium andvariable wavelength filter can be implemented in a single device. Thus,further miniaturization of a device can be anticipated when comparedwith an alternative arrangement which uses a filter.

Furthermore, since no filter is required, the restriction to the widthof wavelength conversion by the bandwidth or the wavelength sweepingwidth is eliminated, and higher speed wavelength conversion can beanticipated.

It is to be noted that, while, in the embodiment described above, anoptical integrated device which includes a semiconductor opticalamplifier as a non-linear medium is used as an example of adaptation ofthe wavelength conversion system described hereinabove, the adaptationof the wavelength conversion system is not limited to this.

For example, it is otherwise possible to form the Mach-Zehnderinterferometer 2 of the wavelength conversion system described abovefrom an optical fiber and form a wavelength conversion apparatus whereina non-linear fiber is used as a non-linear medium.

Such a wavelength conversion apparatus as just described may beconfigured in such a manner as shown in FIG. 3. Referring to FIG. 3, thetwo optical waveguides 2A and 2B of the Mach-Zehnder interferometer 2are formed from polarization maintaining optical fibers (PMF) 84 and 85and the non-linear medium 1 provided on the optical waveguide 2A isformed from a polarization maintaining highly non-linear fiber (PM-HNF)86 while the phase shifter 6 provided on the other optical waveguide 2Bis formed from a polarization controller (PC) 87. Incidentally, in FIG.3, the same components as those in the first embodiment described above(refer to FIG. 1) are assigned with the same reference numerals.

It is to be noted here that polarization controllers (PC) 82 and 83 areprovided for an optical waveguide 80 for inputting signal light ω_(s)therethrough and an optical waveguide 81 for inputting the pumping lightω_(p) therethrough, respectively. Further, the polarization maintainingoptical fibers 84 and 85 serving as the two optical waveguides 2A and 2Bof the Mach-Zehnder interferometer 2 have lengths equal to each other.

[Second Embodiment]

A wavelength conversion system, an optical integrated device and awavelength conversion method according to a second embodiment of thepresent invention are described below with reference to FIGS. 4 and 5(A)to 5(E).

The wavelength conversion system according to the present embodiment issimilar to that of the first embodiment described hereinabove exceptthat two optical waveguides of a Mach-Zehnder interferometer arerespectively provided with non-linear media which have non-linearsusceptibilities (here, third-order non-linear susceptibilities χ⁽³⁾)different from each other but generate gains (linear gains) equal toeach other (including non-linear media which generate no gain) while nobranching ratio adjuster is provided.

In particular, the present wavelength conversion system (apparatus)includes a Mach-Zehnder interferometer 2 having two optical waveguides2A and 2B as seen in FIG. 4, and non-linear media 1A and 1B havingdifferent non-linear susceptibilities from each other and generatingequal gains (including no gains) to each other are provided on the twooptical waveguides 2A and 2B, respectively, of the Mach-Zehnderinterferometer 2 such that light guided through the optical waveguide 2Aof the Mach-Zehnder interferometer 2 and light guide through the otheroptical waveguide 2B of the Mach-Zehnder interferometer 2 interfere witheach other and phase conjugate light ω_(c) outputted as the differencein power (light intensity) between phase conjugate light ω_(cA) producedby the non-linear medium 1A and phase conjugate light ω_(cB) produced bythe non-linear medium 1B is extracted as wavelength conversion light.Incidentally, in FIG. 4, the same components as those in the firstembodiment described above (refer to FIG.1) are assigned with the samereference numerals.

The non-linear susceptibility is a parameter which determines awavelength conversion efficiency, and as the value of the non-linearsusceptibility increases, the wavelength conversion efficiencyincreases.

For example, the non-linear susceptibilities of the non-linear medium(first non-linear medium) 1A provided on the optical waveguide 2A of theMach-Zehnder interferometer 2 and the non-linear medium (secondnon-linear medium) 1B provided on the other optical waveguide 2B may beset such that the non-linear susceptibility of the non-linear medium 1Ais higher (or lower) than that of the non-linear medium 1B.

The first non-linear medium 1A having a higher non-linear susceptibilityis used to generate phase conjugate light ω_(c) to be extracted aswavelength conversion light. Therefore, the first non-linear medium 1Ais called wavelength conversion light producing non-linear medium.Meanwhile, the second non-linear medium 1B having a lower non-linearsusceptibility is used to make the power (light intensity) of the signallight ω_(s) and the pumping light ω_(p) having passed through the firstnon-linear medium 1A and the power (light intensity) of the signal lightω_(s) and the pumping light ω_(p) having passed through the secondnon-linear medium 1B equal to each other. Therefore, the secondnon-linear medium 1B is called output adjusting non-linear medium.

In this instance, when multiplex light of signal light ω_(s) and pumpinglight ω_(p) is branched and introduced into the two optical waveguides2A and 2B of the Mach-Zehnder interferometer 2, the first non-linearmedium 1A provided on the optical waveguide 2A of the Mach-Zehnderinterferometer 2 and the second non-linear medium 1B provided on theother optical waveguide 2B generate phase conjugate lights ω_(cA) andω_(cB) of the signal light ω_(s), respectively.

However, since the first non-linear medium 1A and the second non-linearmedium 1B have different non-linear susceptibilities from each other, adifference appears between the powers (light intensities) of the phaseconjugate lights ω_(cA) and ω_(cB) generated in the non-linear media 1Aand 1B, respectively.

Therefore, even if the light guided through the optical waveguide 2A ofthe Mach-Zehnder interferometer 2 and the light guided through the otheroptical waveguide 2B interfere with each other, the phase conjugatelights ω_(cA) and ω_(cB) do not fully cancel each other but areoutputted.

In this manner, in the present embodiment, the phase conjugate lightω_(c) is extracted as wavelength conversion light making use of thephenomenon that non-linear media having different non-linearsusceptibilities from each other generate phase conjugate lights betweenwhich a power difference (light intensity difference) appears and whichtherefore do not cancel each other.

Particularly, in the present embodiment, since the non-linear media 1Aand 1B provided on the two optical waveguides 2A and 2B of theMach-Zehnder interferometer 2 generate an equal gain, the powers of thelight (light outputs; light intensities) having guided through theoptical waveguides 2A and 2B become equal to each other. Therefore, thebranching ratio adjuster 5 in the first embodiment described hereinaboveis not provided. In other words, the second non-linear medium 1B havinga different non-linear susceptibility is provided on the opticalwaveguide 2B in place of the branching ratio adjuster 5 in the firstembodiment described above so that the powers (light intensities) of thesignal light ω_(s) and the pumping light ω_(p) to be emitted from theoptical waveguides 2A and 2B of the Mach-Zehnder interferometer 2 (twooutput ports of the Mach-Zehnder interferometer 2) may be equal to eachother.

It is to be noted that the configuration of the other part of thepresent embodiment is same as that of the first embodiment describedherein above.

Now, a wavelength conversion method in which such a wavelengthconversion system as described above is described.

First, multiplexed light of signal light ω_(s) and pumping light ω_(p)is branched and introduced into the two optical waveguides 2A and 2B ofthe Mach-Zehnder interferometer 2.

When the multiplexed lights introduced into the two optical waveguides2A and 2B propagate and pass the non-linear media (NLM) 1A and 1B,respectively, phase conjugate lights ω_(cA) and ω_(cB) of the signallight are generated by the non-linear media 1A and 1B, respectively. Inparticular, the phase conjugate light ω_(cA) of the signal light isgenerated by the first non-linear medium 1A provided on the opticalwaveguide 2A of the Mach-Zehnder interferometer 2 while the phaseconjugate light ω_(cB) of the signal light is generated by the secondnon-linear medium 1B provided on the other optical waveguide 2B andhaving a non-linear susceptibility different from that of the firstnon-linear medium 1A.

In the present embodiment, since an equal gain is generated by thenon-linear media 1A and 1B, the powers (light intensities) of the lightguided through the optical waveguide 2A and the light guided through theother optical waveguide 2B become equal to each other. On the otherhand, since the non-linear media 1A and 1B provided on the two opticalwaveguides 2A and 2B of the Mach-Zehnder interferometer 2, respectively,have different non-linear susceptibilities from each other, a differenceappears between the powers (light intensities) of the phase conjugatelights ω_(cA) and ω_(cB) generated by the non-linear media 1A and 1B,respectively. Here, since the non-linear susceptibility of the secondnon-linear medium 1B is lower, the power (light intensity) of the phaseconjugate light ω_(cA) generated by the first non-linear medium 1A islower.

Then, the light guided through the optical waveguide 2A and the lightguided through the optical waveguide 2B interfere with each other at thecoupler 4 (at the branching ratio of 1:1).

In the present embodiment, since a difference is provided between thepowers (light intensities) of the phase conjugate lights ω_(cA) andω_(cB) generated by the non-linear media 1A and 1B, the phase conjugatelights ω_(cA) and ω_(cB) do not fully cancel each other but areoutputted individually from the two ports of the coupler 4. Meanwhile,the signal lights ω_(s) and the pumping lights ω_(p) having propagatedthrough the optical waveguides 2A and 2B cancel each other at thecoupler 4 and are not outputted from one of the ports of the coupler 4.Therefore, from the one of the ports of the coupler 4, only the phaseconjugate light ω_(c) (=ω_(cA)−ω_(cB)) is outputted, and this isextracted as wavelength conversion light. Consequently, the signal lightω_(s) is converted into the phase conjugate light ω_(c).

In the present embodiment, phase shifting is performed by the phaseshifter 6 provided on the optical waveguide 2B so that the phases of thelights having propagated through the two optical waveguides 2A and 2Bmay coincide with each other.

Now, an optical integrated device which uses the wavelength conversionsystem described hereinabove is described with reference to FIGS. 5(A)to 5(E).

It is to be noted that FIG. 5(B) is a schematic sectional view takenalong line B-B′ of FIG. 5(A); FIG. 5(C) is a schematic sectional viewtaken along line C-C′ of FIG. 5(A); FIG. 5(D) is a schematic sectionalview taken along line D-D′ of FIG. 5(A); and FIG. 5(E) is a schematicsectional view taken along line E-E′ of FIG. 5(A).

Referring first to FIG. 5(A), the present optical integrated device isformed as a monolithic integrated device wherein, as a wavelengthconversion section (Mach-Zehnder interferometer 2 and non-linear media1A and 1B of the wavelength conversion system described hereinabove) forremoving signal light ω_(s) and pumping light ω_(p) to selectivelyextract phase conjugate light ω_(c) a post-stage Mach-Zehnderinterferometer 20, a first semiconductor optical amplifier (first SOA)10A used as the first non-linear medium 1A of the wavelength conversionsystem described hereinabove, and a second semiconductor opticalamplifier (second SOA) 10B used as the second non-linear medium 1B ofthe wavelength conversion system described hereinabove are integratedmonolithically. Incidentally, in FIGS. 5(A) to 5(E), the same componentsas those in the first embodiment described above (refer to FIGS. 2(A) to2(E)) are assigned with the same reference numerals.

The post-stage Mach-Zehnder interferometer 20 includes an input sidecoupler 21, an output side coupler 22, and two optical waveguides (slabwaveguides) 23 and 24 for interconnecting the input side coupler 21 andthe output side coupler 22 as seen in FIG. 5(A).

Each of the first SOA 10A and the second SOA 10B is provided on acorresponding one of the two optical waveguides 23 and 24 which form thepost-stage Mach-Zehnder interferometer 20 as shown in FIG. 5(A). Each ofthe first SOA 10A and the second SOA 10B includes an electrode 10AX or10BX provided on an optical waveguide including an active layer made ofa gain medium which generates a third-order non-linear phenomenon suchthat electric current (control current) can be injected into the activelayer through the electrode 10AX or 10BX.

Here, the non-linear susceptibility of the first SOA 10A is set higherthan that of the second SOA 10B. More particularly, the first SOA 10Aand the second SOA 10B may be configured in the following manner.

(1) The first SOA 10A is formed as a bulk SOA while the second SOA 10Bis formed as a strained quantum well SOA. (2) The first SOA 10A isformed as a bulk SOA while the second SOA 10B is formed as a quantum dotSOA. (3) The first SOA 10A is formed as a non-strained quantum well SOAwhile the second SOA 10B is formed as a strained quantum well SOA. (4)The first SOA 10A is formed as anon-strained quantum well SOA while thesecond SOA 10B is formed as a quantum dot SOA.

It is to be noted that the non-linear susceptibility of the first SOA10A may be set lower than that of the second SOA 10B. In this instance,the first SOA 10A and the second SOA 10B may be configured in thefollowing manner.

(1) The first SOA 10A is formed as a strained quantum well SOA while thesecond SOA 10B is formed as a bulk SOA. (2) The first SOA 10A is formedas a quantum dot SOA while the second SOA 10B is formed as a bulk SOA.(3) The first SOA 10A is formed as a strained quantum well SOA while thesecond SOA 10B is formed as a non-strained quantum well SOA. (4) Thefirst SOA 10A is formed as a quantum dot SOA while the second SOA 10B isformed as a non-strained quantum well SOA.

In this manner, the first SOA 10A and the second SOA 10B may be formedso as to include different active layers having different non-linearsusceptibilities from each other. For example, the first SOA 10A may beformed so as to include active layers composed of any one type ofconfigurations chosen from group including bulk, non-strained quantumwell, strained quantum well and quantum dot layers while the second SOA10B is formed so as to include one of such active layers as mentionedabove which is different from that of the first SOA 10A. It is to benoted that the first SOA 10A and the second SOA 10B may otherwise beconfigured so as to include active layers of the same type but havingdifferent non-linear susceptibilities from each other.

Further, the present optical integrated device is configured such thatthe first SOA 10A and the second SOA 10B generate an equal gain so thatthe powers (light intensities) of the signal light ω_(s) and the pumpinglight ω_(p) to be emitted from the two optical waveguides 23 and 24which form the post-stage Mach-Zehnder interferometer 20 may be equal toeach other.

Then, the multiplex light of the signal light ω_(s) and the pumpinglight ω_(p) is introduced into the two optical waveguides 23 and 24which form the post-stage Mach-Zehnder interferometer 20, and the firstSOA 10A and the second SOA 10B generate phase conjugate lights ω_(cA)and ω_(cB) of the signal light ω_(s) respectively. Then, the lightpropagated through the optical waveguide 23 and the light propagatedthrough the other optical waveguide 24 interfere with each other, andthe phase conjugate light ω_(c) (=ω_(cA)−ω_(cB)) is extracted aswavelength conversion light.

Further, in the present embodiment, a phase shifter 60 (phasecompensator) is provided as the phase shifter 6 of the wavelengthconversion system described hereinabove on one of the two opticalwaveguides 23 and 24 of the post-stage Mach-Zehnder interferometer 20(here, on the optical waveguide 24) as seen in FIG. 5(A) such that phaseshifting can be performed so that the phases of the lights havingpropagated through the two optical waveguides 23 and 24 may coincidewith each other. Here, the phase shifter 60 includes a phase shiftingelectrode 60A (the region in which the electrode is provided is referredto as current injection region) provided on one of the opticalwaveguides of the post-stage Mach-Zehnder interferometer 20 (here, onthe optical waveguide 24) such that current is injected into the opticalwaveguide 24 through the phase shifting electrode 60A to perform thephase adjustment. It is to be noted that, if it is possible to adjustthe phases to each other by some other measures such as adjustment ofthe lengths of the two optical waveguides 23 and 24 of the Mach-Zehnderinterferometer 20, then the phase shifter need not be provided.

Now, a method of producing the optical integrated device according thepresent embodiment is described suitably with reference to FIGS. 5(A) to5(E). Incidentally, in FIGS. 5(A) to 5(E), the same components as thosein the first embodiment described above (refer to FIGS. 2(A) to 2(E))are assigned with the same reference numerals.

First, an n-InP cladding layer 61 (for example, of a thickness of 1 μmor less), a lower side SCH layer 62 (InGaAsP layer; optical guide layer;for example, of a light emission wavelength of 1.15 μm; for example, ofa thickness of 50 nm), a strained quantum well active layer 63 (having,for example, six InGaAs well layers; for example, of a strain amount of+0.8%; for example, of a thickness of 100 nm), an upper side SCH layer64 (InGaAsP layer; optical guide layer; for example, of a light emissionwavelength of 1.15 μm; for example, of a thickness of 50 nm), and ap-InP cladding layer 65 (for example, of a thickness of 300 nm) aregrown on an n-type InP substrate (n-InP substrate) 100, for example, bymetal organic chemical vapor phase epitaxy (MOVPE) to form a stackedstructure including the strained quantum well active layer 63 (first SOAactive layer) which forms the first SOA 10A as seen FIG. 5(E).

Then, an SiO₂ mask (dielectric mask) is formed only at a portion atwhich the first SOA 10A is to be formed (refer to FIG. 5(A), and leavingthis portion, the layers are removed, for example, by wet etching untilthe n-InP cladding layer 61 is exposed (that is, the layers from thep-InP cladding layer 65 on the front face side to the lower side SCHlayer 62 are removed).

Then, as seen in FIGS. 5(B) to 5(E), at the portion from which thelayers are removed, a lower side SCH layer 62 (InGaAsP layer; opticalguide layer; for example, of a light emission wavelength of 1.15 μm; forexample, of a thickness of 50 nm), a non-strained quantum well activelayer 72 (having three InGaAs well layers; for example, of a thicknessof 70 nm), an upper side SCH layer 64 (InGaAsP layer; optical guidelayer; for example, of a light emission wavelength of 1.15 μm; forexample, of a thickness of 50 nm), and a p-InP cladding layer 65 (forexample, of a thickness of 300 nm) are grown, for example, by MOVPE toform a stacked structure including the non-strained quantum well activelayer 72 (second SOA active layer) which forms the second SOA 10B.

Then, an SiO₂ mask is formed only at portions at which the first SOA 10Aand the second SOA 10B are to be formed [refer to FIG. 5(A)], andleaving the portions, the layers are removed, for example, by wetetching until the n-InP cladding layer 61 is exposed (that is, thelayers from the p-InP cladding layer 65 on the front surface side to thelower side SCH layer 62 are removed).

After the etching is performed in this manner, as seen in FIGS. 5(B) to5(E), at the portions from which the layers are removed, a lower sideSCH layer 62 (InGaAsP layer; optical guide layer; for example, of alight emission wavelength of 1.15 μm; for example, of a thickness of 50nm), a waveguide core layer 66 (InGaAsP layer; for example, of a lightemission wavelength of 1.3 μm; for example, of a thickness of 200 nm),an upper side SCH layer 64 (InGaAsP layer; optical guide layer; forexample, of a light emission wavelength of 1.15 μm; for example, of athickness of 50 nm), and a p-InP cladding layer 65 (for example, of athickness of 300 nm) are butt joint grown, for example, by MOVPE or thelike to form a stacked structure including the waveguide core layer 66.

Then, an SiO₂ mask is formed at portions at which the optical waveguides23, 24, 41 and 42 are to be formed, at portions at which the MMIcouplers 21 and 22 are to be formed and at portions at which the SOAs10A and 10B are to be formed [refer to FIG. 5(A)], and then dry etchingsuch as, for example, ICP-RIE (Inductively Coupled Plasma Reactive IonEtching) is performed to form a waveguide mesa structure, for example,of a height of 1.5 μm and a width of 1.5 μm as seen in FIGS. 5(A) to5(E).

Then, a p-InP current block layer 67 (first current block layer) and ann-InP current block layer 68 (second current block layer) are grown onthe opposite sides of the mesa structure, for example, by MOVPE or thelike to form a current constriction structure as seen in FIGS. 5(A) to5(E).

After the current constriction structure is formed in this manner, theSiO₂mask is removed, and ap-InP cladding layer 69 (for example, of athickness of 3 μm) and an InGaAsP contact layer 70 (for example, of alight emission wavelength of 1.3 μm; for example, of a thickness of 100nm) are grown at an upper portion thereby to complete the epitaxialgrowth as seen in FIGS. 5(B) to (E).

From the wafer on which the epitaxial growth is completed in thismanner, the InGaAsP contact layer 70 is removed except at the portionsat which the first SOA 10A and the second SOA 10B are to be formed andat the portion at which the phase shifter 60 is to be formed, and then aSiO₂ film 71 is formed as seen in FIGS. 5(A) to (E). Thereafter, theSiO₂ film 71 at the portions at which the first SOA 10A and the secondSOA 10B are to be formed and at the portion at which the phase shifter60 is to be formed (at the portions at which the contact layer 70remains) is removed, and p side electrodes 60A, 10AX and 10BX are formedon the InGaAsP contact layer 70. Meanwhile, an n side electrode 73 isformed on the rear face of the substrate.

Finally, anti-reflection coating is applied to the opposite side facesof the device after cleavage thereby to complete the optical integrateddevice.

Accordingly, according to the wavelength conversion system, opticalintegrated device and wavelength conversion method of the presentembodiment, similar advantages to those of the first embodimentdescribed hereinabove are achieved. Further, since the branching ratioadjustment section in the first embodiment described hereinabove neednot be provided, further miniaturization can be anticipated.

It is to be noted that, while, in the present embodiment, the non-linearmedia 1A and 1B generate gains equal to each other (including a casewherein no gains are generated), the non-linear media are not limited tothem. For example, non-linear media having different gains from eachother may be provided while the branching ratio adjuster in the firstembodiment described above is provided.

[Others]

It is to be noted that the stacked structure and the production methodof an optical integrated device are not limited to those of theembodiments described hereinabove. Further, while the embodimentsdescribed hereinabove use a mesa structure of the buried type,alternative structures such as a high mesa structure which does notinvolve such burying or a ridge structure may be used. Also in thisinstance, similar advantages to those described hereinabove can beachieved.

The present invention is not limited to the embodiment specificallydescribed above, and variations and modifications can be made withoutdeparting from the scope of the present invention.

1. A wavelength conversion system, comprising: a Mach-Zehnderinterferometer including two optical waveguides; a non-linear mediumprovided on one of the two optical waveguides; and a branching ratioadjustment section for adjusting the branching ratio of multiplexedlight produced by multiplexing signal light and pumping light so thatthe powers of the signal light and the pumping light which are to beemitted from the two optical waveguides are equal to each other; themultiplexed light whose branching ratio is adjusted by said branchingratio adjustment section being introduced into the two opticalwaveguides such that said non-linear medium generates phase conjugationlight of the signal light and the light guided through the one opticalwaveguide and the light guided through the other one of the two opticalwaveguides interfere with each other so that the phase conjugation lightis extracted as wavelength conversion light.
 2. A wavelength conversionsystem, comprising: a Mach-Zehnder interferometer including two opticalwaveguides; a first non-linear medium provided on one of the two opticalwaveguides; and a second non-linear medium provided on the other one ofthe two optical waveguides and having a non-linear susceptibilitydifferent from that of said first non-linear medium; multiplexed lightproduced by multiplexing signal light and pumping light being branchedand introduced into the two optical waveguides such that said first andsecond non-linear media individually generate phase conjugation light ofthe signal light and the light guided through the one optical waveguideand the light guided through the other one of the two optical waveguidesinterfere with each other so that the phase conjugation light isextracted as wavelength conversion light.
 3. The wavelength conversionsystem as claimed in claim 2, wherein said first non-linear medium andsaid second non-linear medium generate gains equal to each other.
 4. Thewavelength conversion system as claimed in claim 2, further comprising abranching ratio adjustment section for adjusting the branching ratio ofmultiplexed light produced by multiplexing signal light and pumpinglight so that the powers of the signal light and the pumping light whichare to be emitted from the two optical waveguides are equal to eachother.
 5. The wavelength conversion system as claimed in claim 1,further comprising a phase shifter provided on at least one of the twooptical waveguides.
 6. The wavelength conversion system as claimed inclaim 1, wherein said non-linear medium is a semiconductor opticalamplifier.
 7. The wavelength conversion system as set forth in claim 2,wherein each of said first and second non-linear mediums is asemiconductor optical amplifier.
 8. The wavelength conversion system asset forth in claim 1, wherein said non-linear medium is a non-linearoptical fiber.
 9. The wavelength conversion system as set forth in claim2, wherein each of said first and second non-linear mediums is anon-linear optical fiber.
 10. The wavelength conversion system asclaimed in claim 1, wherein said branching ratio adjustment section isformed from a coupler.
 11. The wavelength conversion system as claimedin claim 1, wherein said branching ratio adjustment section is formedfrom a Mach-Zehnder interferometer.
 12. An optical integrated device,comprising: a pre-stage Mach-Zehnder interferometer including twooptical waveguides for branching and guiding multiplexed light producedby multiplexing signal light and pumping light; a phase shifter providedon at least one of the two optical waveguides which form said pre-stageMach-Zehnder interferometer; a post-stage Mach-Zehnder interferometerincluding an input side coupler, an output side coupler, and two opticalwaveguides for connecting said input side coupler and said output sidecoupler to each other; and a semiconductor optical amplifier provided onone of the two optical waveguides which form said pre-stage Mach-Zehnderinterferometer; the multiplexed light whose branching ratio is adjustedby said pre-stage Mach-Zehnder interferometer and said phase shifter sothat the powers of the signal light and the pumping light to beindividually emitted from the two optical waveguides which form saidpost-stage Mach-Zehnder interferometer are equal to each other beingintroduced into the two optical waveguides which form said post-stageMach-Zehnder interferometer such that said semiconductor opticalamplifier generates phase conjugation light of the signal light and thelight guided through the one optical waveguide which forms saidpost-stage Mach-Zehnder interferometer and the light guided through theother one of the two optical waveguides which form said post-stageMach-Zehnder interferometer interfere with each other so that the phaseconjugation light is extracted as wavelength conversion light.
 13. Anoptical integrated device, comprising: a Mach-Zehnder interferometerincluding an input side coupler, an output side coupler, and two opticalwaveguides for connecting said input side coupler and said output sidecoupler to each other; a first semiconductor optical amplifier providedon one of the two optical waveguides; and a second semiconductor opticalamplifier provided on the other one of the two optical waveguides andhaving non-linear susceptibility different from that of said firstsemiconductor optical amplifier; multiplexed light produced bymultiplexing signal light and pumping light being branched andintroduced into the two optical waveguides such that said first andsecond semiconductor optical amplifiers individually generate phaseconjugation light of the signal light and the light guided through theone optical waveguide and the light guided through the other one of thetwo optical waveguides interfere with each other so that the phaseconjugation light is extracted as wavelength conversion light.
 14. Theoptical integrated device as claimed in claim 13, wherein said first andsecond semiconductor optical amplifiers generate gains equal to eachother.
 15. The optical integrated device as claimed in claim 13, whereinsaid first and second semiconductor optical amplifiers include differentactive layers having non-linear susceptibilities different from eachother.
 16. The optical integrated device as claimed in claim 15, whereinsaid first semiconductor optical amplifier includes an active layerselected from among a bulk layer, a non-strained quantum well layer, astrained quantum well layer and a quantum dot layer, and said secondsemiconductor optical amplifier includes an active layer selected fromamong the bulk layer, non-strained quantum well layer, strained quantumwell layer and quantum dot layer but different in type from that of theactive layer of said first semiconductor optical amplifier.
 17. Theoptical integrated device as claimed in claim 13, wherein said first andsecond semiconductor optical amplifiers include active layers of thesame type having non-linear susceptibilities different from each other.18. The optical integrated device as claimed in claim 12 furthercomprising a phase shifter provided on at least one of the two opticalwaveguides which form said post-stage Mach-Zehnder interferometer. 19.The optical integrated device as claimed in claim 13, further comprisinga phase shifter provided on at least one of the two optical waveguideswhich form said Mach-Zehnder interferometer.
 20. The optical integrateddevice as claimed in claim 12, wherein each of said input side couplerand said output side coupler is a multi-mode interference coupler. 21.The optical integrated device as claimed in claim 12, wherein each ofsaid input side coupler and said output side coupler is a directionalcoupler.
 22. A wavelength conversion method, comprising the steps of:adjusting a branching ratio of multiplexed light produced bymultiplexing signal light and pumping light emitted from two opticalwaveguides which form a Mach-Zehnder interferometer so that the powersof the signal light and pumping light are equal to each other;introducing the multiplexed light whose branching ratio is adjusted intothe two individual optical waveguides; generating phase conjugationlight of the signal light by means of a non-linear medium provided onone of the two optical waveguides; and causing the light guided throughthe one optical waveguide with a gain generated by the non-linear mediumand the light guided through the other one of the two optical waveguidesto interfere with each other so that the phase conjugation light isextracted as wavelength conversion light.
 23. A wavelength conversionmethod, comprising the steps of: introducing multiplexed light producedby multiplexing signal light and pumping light into two opticalwaveguides which form a Mach-Zehnder interferometer; generating phaseconjugation light of the signal light by means of a first non-linearmedium provided on one of the two optical waveguides; generating phaseconjugation light of the signal light by means of a second non-linearmedium provided on the other one of the two optical waveguides andhaving a non-linear susceptibility different from that of the firstnon-linear medium; and causing the light guided through the one opticalwaveguide and the light guided through the other optical waveguide tointerfere with each other so that the phase conjugation light isextracted as wavelength conversion light.
 24. The wavelength conversionmethod as claimed in claim 23, wherein the second non-linear medium isformed from a medium which generates a gain equal to that of the firstnon-linear medium.
 25. The wavelength conversion method as claimed inclaim 22, wherein the phases of the light guided through one of the twooptical waveguides and the light guided through the other one of the twooptical waveguides are adjusted by a phase shifter provided on at leastone of the two optical waveguides.